JPH01281644A - X-ray tube having molybdenum target - Google Patents

Abstract

PURPOSE: To prevent the change of X-ray intensity by forming a molybdenum target out of molybdenum alloy containing vanadium. CONSTITUTION: A target 30 hit by an electron beam 13 is formed of at least 0.5wt.% vanadium alloy or vanadium doped molybdenum, so that the deterioration or cracking of the target 30, when beated by the electron beam 13, can be greatly delayed or reduced. In this way, the significant decrease of X-ray intensity and the change of X-ray intensity in an X-ray irradiation region, which are specially important problems with a X-ray tube used for a mammography, can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an X-ray tube, in particular an X-ray tube for mammography, and more particularly to an anode target made of molybdenum.

In prior art x-ray tubes, x-rays are obtained by bombarding with electrons a target supported on an anode or formed by the anode itself. On the target, a small area is struck by electrons to form an x-ray source. The characteristics of the X-rays depend on the incident electron beam and the properties of the material forming the target.

Molybdenum targets are widely used in mammography X-ray tube anodes.

The effectiveness of molybdenum targets in mammography lies in the fact that the energy spectrum of the X-rays produced is particularly suited for such examinations.

This is because breasts do not absorb much X-rays, and the abnormality they are looking for has a density very similar to that of the surrounding tissue.

The contrast appearing between the abnormality and the surrounding tissue in the X-ray image is significantly improved if the X-rays used have a narrow energetic band containing the characteristic X-rays of molybdenum.

Generally, molybdenum targets are obtained by sintering molybdenum powder. Most often, the anode itself consists of sintered molybdenum powder. The target is part of this anode. There are two types of anode: fixed type and rotating type. Using CVD and other standard methods, including electrodeposition, a molybdenum layer can be deposited on the anode either over the entire surface of the anode or, in the case of a rotating anode, along the focal line, for example. A target can be formed on that part.

Problem to be Solved by the Invention When a molybdenum target is repeatedly struck by electrons, thermomechanical stress is generated in the target, causing cracks in the molybdenum. These cracks increase in number and size as they are exposed to electron beams. The presence of cracks reduces the x-ray efficiency of the x-ray tube. This can be explained by the fact that electrons entering the cracks generate X-rays, most of which are absorbed by the anode itself.

When the efficiency of X-rays decreases, the following two problems appear in particular. These two issues are particularly important in mammography.

1. Decrease in image quality. This is explained by the increased motion blur due to the required longer exposure time as a result of the reduced efficiency of the x-rays, and the non-uniform density of the image within the x-ray irradiation area. .

2. The radiation dose received by the patient increases due to the so-called non-reciprocal deviation of the film/screen pair used in mammography. In fact, due to the non-reciprocity of the exposed material (film/screen), the number of photons required to sensitize the film increases with increasing exposure time.

For example, in a direction close to the direction of maximum X-ray intensity, the X-ray intensity decreases by about 20% after normal operation of the X-ray tube, and by about 40% in a plane close to the anode plane.

The present invention includes a fixed type or rotating type anode, which is free from the above-mentioned cracking problem or
This problem appears to a much lesser extent after much longer operating times than with conventional molybdenum targets, particularly used in, but not limited to, mammography. Regarding wire tubes.

Means for solving the problem This is obtained by doping molybdenum. This doping is believed to have the effect of strengthening the bonds between particles and the molybdenum particles themselves, making molybdenum more elastic.

According to the present invention, a cathode that generates an electron beam;
an anode having a molybdenum target whose surface is struck by the electron beam and constitutes an X-ray source;
X wire tube, characterized in that the molybdenum target is formed of a molybdenum alloy containing vanadium.
A wire tube is provided.

Furthermore, it has been confirmed that doping molybdenum with tungsten has significant advantages in the case of X-ray tubes used in mammography. The advantage of this is that the characteristic x-rays of vanadium, which are generated by electron bombardment, can be completely removed with standard filters for x-rays generated by x-ray tubes. As a result, the X-ray spectrum remains unchanged in the presence of vanadium.

The invention will be better understood from the following description with reference to the accompanying drawings. Note that the drawings merely show examples and do not limit the present invention.

The embodiment drawings show the X of the present invention used especially for mammography
An example of a wire tube 1 is shown. However, the embodiments are not limited to this.

The X-ray tube 1 is housed within the casing 2 in the same manner as in the prior art. This X-ray tube 1 is equipped with a closed envelope 3 made of glass, for example, inside which a cathode 4 is placed.
and an anode 5. In this embodiment, the anode 5 is a rotating anode whose overall shape is a disk, and is supported at the center of the disk by a support shaft 7 fixed to a rotor 8. The rotor 8 is arranged along the symmetry axis 9 of the disk-shaped anode 5. This rotor 8 is supported by a support lO fixed to the envelope 3 in a conventional manner. A stator 11 is arranged outside the envelope 3 and rotates the rotor 8 and, as a result, the anode 5 about the axis of symmetry 9. The cathode 4 is also supported by the envelope 3 in a conventional manner, and faces the end 12 of the disc-shaped anode 5.

The cathode 4 generates an electron beam 13 which strikes a target 30 formed on the end 12 in this embodiment. Since the anode 5 is a rotating anode, the target 30 forms a focal line around the axis of symmetry 9 along the end 12 . The electron beam 13 strikes a limited portion 15 of the target. This portion 15 is called the focal point and constitutes the X-ray source.

According to one feature of the invention, the target 30 struck by the electron beam 13 consists of molybdenum alloyed or doped with at least 0.5% by weight of vanadium. In this way, the aforementioned deterioration and cracking caused by the target 30 being hit by the electron beam 13 can be significantly delayed or reduced. A significant improvement is observed when the amount of vanadium exceeds 0.5% by weight. The optimum condition is that vanadium is mixed in a proportion of 2.5 to 3.5% by weight relative to molybdenum. When vanadium is in excess (e.g. more than 7% by weight), the X-rays emitted from the X-ray tube 1 are filtered so that the energy spectrum of the X-rays covers a relatively narrow band containing the characteristic X-rays of molybdenum. After that, the intensity of the X-rays may drop to a non-negligible level.

In this embodiment, this filtering operation is performed at the output window 33. The X-rays first exit from the X-ray tube 1 through the first output window 32 which absorbs almost no X-rays, and then exit from the casing 2 through the output window 33. The first output window 32 is made of beryllium, for example, as in the conventional case,
The second output window 33 is made of molybdenum.

Targeted end 12 can be manufactured in a variety of ways.

- the target 30 can for example be constituted directly by the anode 5 itself; This anode 5 is made by doping molybdenum with vanadium in the above ratio.

In this case, the anode 5 can be manufactured by a known method of sintering molybdenum mixed with vanadium, for example.

- However, the target 30 can also be in the form of a layer 40 of a molybdenum-vanadium mixture.

This layer 40 is deposited at a desired position on the disc-shaped anode 5, which constitutes a substrate made of, for example, molybdenum in this example. The molybdenum-vanadium layer 40 is deposited to a thickness E of several microns by known methods, for example electrodeposition or CVD.

In the CVD method, a vapor phase mixture of molybdenum and vanadium is configured to a desired ratio at the location where the deposition is to be performed, taking into account the kinetic characteristics of the reaction (temperature, pressure, gas introduction rate, etc.).

The X-rays emitted from the casing 2 pass through the collimator 41 in the conventional manner and constitute effective X-rays 43 within the boundaries 45, 46. First boundary 4 located on the cathode 4 side
5 is generally located on the side of the patient's (not shown) rib cage in mammography. On the other hand, the second boundary 46 on the side of the anode 5 is located on the side of the tip of the breast to be examined, that is, the nipple. Second boundary 46 forms a relatively small angle α of about 2 degrees with the anode surface defined by end 12 . The efficiency reduction described above is characteristic of the x-rays between boundaries 45 and 46. That is, the intensity of the x-rays increases from the second boundary 46 to the first boundary 45 to compensate for differences in the thickness of the breast (not shown) between the nipple and the rib cage.

Curve 50 is an example of an X-ray intensity change between two boundaries 45,46. This curve 50 represents the rate of change of the maximum intensity of the X-rays in the direction along the first boundary 45.

Following this curve 50, at the first boundary 45 10
The intensity of the X-rays which is 0% reaches approximately 65% at the second boundary 46 while gradually increasing the rate of decrease.

This is because the target has few cracks either because the target is composed of molybdenum doped with vanadium according to the invention or because it is made of molybdenum that has hardly been bombarded with electrons. It corresponds to the case.

A second curve 51, indicated by a dotted line, shows the change in the intensity of the X-rays under the same conditions as described above, but with a conventional molybdenum target. When this target is hit with electrons and degrades, it cracks. X at first boundary 45
The intensity of the line is about 80%, i.e. the intensity is about 20
% decrease. The second curve 51 shows that the intensity of the X-rays decreases more sharply than in the first case and reaches the second boundary 4.
6, it reaches 25%.

Effects of the invention This shows that using a target 30 made of molybdenum alloyed with vanadium or doped with vanadium according to the invention prevents cracking of the target due to repeated electron bombardment and at the same time can be used in mammography. This shows that it is possible to prevent a large drop in X-ray intensity, which is a particularly important problem for X-ray tubes, and to prevent changes in X-ray intensity within the X-ray irradiation range.

[Brief explanation of the drawing]

The drawing is a schematic diagram of an X-ray tube of the invention. (Main reference numbers) 1...X-ray tube, 2...casing, 3...envelope, 4...cathode, 5...anode,
7. Support shaft, 8. Rotor,
10...Support body, 11...Stator, 12
... End, 13. Electron beam, 30. Target, 32. 33. Output window, 40. Molybdenum-vanadium layer, 41. Collimator, 43. Effective X-ray, 45.
46... Boundary Patent Applicant General Electric Sageney Air Varnish, Ah.

Claims (7)

[Claims] (1) An X-ray tube comprising a cathode that generates an electron beam and an anode having a molybdenum target whose surface is struck by the electron beam to constitute an X-ray source, the molybdenum target having at least 0.5 X-ray tube, characterized in that it is made of a molybdenum alloy containing a proportion of vanadium in the proportion of % by weight. (2) The X-ray tube according to claim 1, wherein the molybdenum alloy contains 2.5 to 3.5% by weight of vanadium. (3) The X-ray tube according to claim 1 or 2, wherein the target is formed directly by the anode, and the anode is manufactured by sintering molybdenum powder mixed with vanadium. (4) An X-ray tube according to claim 1 or 2, characterized in that the anode comprises a substrate, on which the target is deposited in the form of a layer. (5) The X-ray tube according to any one of claims 1 to 4, wherein the layer forming the target is deposited by a CVD method. (6) The X-ray tube according to any one of claims 1 to 4, wherein the anode is a rotating anode. (7) The X according to any one of claims 1 to 6, which constitutes an X-ray tube of a mammography device.
wire tube.